Body Wave Energy Investigations in Ambient Seismic Noise

The microseismic frequency band contains a large amount of coherent energy, which has generated much interest recently because of its use in surface wave tomography as well as its apparent link to temporal changes in ocean wave behavior. Although the dominant component of this energy travels as surface waves, body waves also contribute a significant portion at slightly higher frequencies. The surface wave noise appears to be generated by ocean wave interaction along the coastline, however, several studies have indicated that the body wave energy may originate from the deep ocean suggesting the possibility of different mechanisms for the generation of surface and body wave noise. Further exploration of the nature of body wave noise is necessary to better understand how and where it is generated, and what additional information we might gain from it. We investigate source locations of body wave noise generation using a back-projection technique similar to those used to study earthquake rupture properties. We use data from two arrays, YKA and ILAR, both of which are small aperture seismic arrays of the International Monitoring System. Array analysis provides a powerful tool for placing constraints on the direction and mode of incoming coherent energy. We examine a year's worth of data in 10 second time windows and back-project each window to a grid of potential source locations to determine the time and location with the greatest beam power. Preliminary results from YKA show a strong seasonal signal. Source locations from January to May concentrated in an area in the middle of the northern Pacific, consistent with independent results from a traditional frequency-wavenumber analysis and supportive of the possibility of a persistent deep ocean generation for microseismic P-waves. From June to September, source locations shift to the northwestern Atlantic, while October to November shows a more diffuse area of sources. We will compare our results with satellite observations of significant wave height as well as the wave-wave interaction parameter from reanalysis models.